This invention relates to a key telephone system and more particularly to a lockout circuit for a paging system of such a telephone system.
Prior art intercom cards contain only a single relay for paging circuits. When the key button or switch is depressed, ground is connected to the paging circuit relay thereby causing this relay to activate. A contact of the relay connects voice signals developed in the telephone subset circuit to the input of a paging amplifier. The amplifier input is common to all such paging circuits in multiple card systems. Amplifier activation is accomplished by either a second paging relay contact connecting ground to a common amplifier keying circuit or by arranging the common amplifier keying path as a continuation of each card's paging relay circuit.
One shortcoming of these prior art devices occurs when two cards have their respective paging relays keyed at the same time. This condition connects the subset circuits of both cards to the common paging amplifier input. If the parties using both cards now attempt to make a paging annoucement, the result will be a garbled mixture amplified over the loud speaker system. Since each party is using a different card and there is no method to indicate what one party or the other is about to do, therefore, simultaneous but independent paging is possible.
Another shortcoming of these prior art devices occurs when a key line is shorted to ground due to circuit faults or physical cable damage. This permanently activates the paging relay of the card in question resulting in the "broadcasting" or paging of a conversation often without the knowledge of the parties involved.